The invention relates to a method for preparing pure carboxylic acids from their reaction solutions, wherein cobalt carbonyl catalysts, such as especially Co.sub.2 (CO).sub.8 or corresponding ionic cobalt carbonyl compounds, are contained. Such cobalt carbonyl catalysts contain valuable cobalt, which it is desirable to recover. Furthermore, the cobalt catalysts make it difficult to produce the carboxylic acids in pure form and they form contaminants in the target products.
By known methods of carbonylation with cobalt carbonyl catalysts, especially dicobalt octacarbonyl, a series of carboxylic acids can be prepared by the insertion of one or, in some cases, two molecules of carbon monoxide and, if desired, additional molecular components, such as for example aryl acetic acids, especially phenylacetic acid, malonic acid and its esters, phenylpyruvic acid, and numerous others. In this case comparatively small amounts of the cobalt catalyst need to be separated.
In the case of the preparation of acetylaminocarboxylic acids, especially N-acetyl-DL-phenylalanine, however, very large amounts of cobalt carbonyl catalyst are required.
Especially, therefore, the invention relates to a method for preparing pure N-acetyl amino acids, including particularly N-acetyl-DL-phenylalanine, by the removal of cobalt carbonyl catalysts from the reaction solution while at the same time recovering the cobalt they contain.
N-acetyl amino carboxylic acids, especially N-acetylalpha-aminocarboxylic acids, can be obtained in a manner known in itself, described in German publication OS No. 2,364,039, from the corresponding organic halides by transposition with hydrogen and carbon monoxide in the presence of cobalt carbonyl catalysts and an amide. In this process large amounts of cobalt carbonyl catalysts are necessary. The German publication, however, specifies no technically useful method of selectively and completely removing the large amounts of the catalyst and recovering the cobalt. Furthermore, the German publication does not disclose how the acetyl aminocarboxylic acids are isolated and can be separated from the residues of the cobalt in the necessary manner. Accordingly, the method taught by the German publication is impractical in the disclosed form.
The problem existed of isolating carboxylic acids from their reaction solutions containing cobalt carbonyl catalysts, freeing them insofar as possible from residues of cobalt compounds, and recovering the cobalt contained in the cobalt carbonyl catalysts in a form which permits the reuse of the cobalt.
The problem consisted especially in preparing N-acetylalpha-aminocarboxylic acids, especially N-acetyl-DL-phenylalanine, in pure form with minimal losses from their reaction solutions containing cobalt carbonyl catalysts, separating the cobalt of the catalyst from the target product, removing it as completely as possible from the reaction solution, and isolating it in recoverable form.
This problem is aggravated by the requirement of high purity for the acetyl amino acids, especially insofar as they are to be used for pharmaceutical purposes, and especially again insofar as they are intended for use as starting substances for enzymatic deracemization. The problem is further aggravated by incomplete formation of the N-acetyl aminocarboxylic acid and by the large percentages of byproducts as well as large amounts of chlorides in the reaction solution.